This invention is generally in the area of the Fischer-Tropsch synthesis.
The majority of fuel today is derived from crude oil. Crude oil is in limited supply, and fuel derived from crude oil tends to include nitrogen-containing compounds and sulfur-containing compounds, which are believed to cause environmental problems such as acid rain.
Although natural gas includes some nitrogen- and sulfur-containing compounds, methane can be readily isolated in relatively pure form from natural gas using known techniques. Many processes have been developed which can produce fuel compositions from methane. Most of these process involve the initial conversion of methane to synthesis gas (xe2x80x9csyngasxe2x80x9d).
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes a broad spectrum of products, ranging from methane to wax, with a significant amount of hydrocarbons in the distillate fuel range (C5-20).
Methane tends to be produced when chain growth probabilities are low, which is generally not preferred. Heavy products with a relatively high selectivity for wax are produced when chain growth probabilities are high. The wax can be processed to form lower molecular weight products, but this processing often results in undesired formation of C1-4 products. Paraffinic Fischer-Tropsch products tend to be mostly linear, and tend to have relatively low octane values, high cetane numbers, relatively high pour points and relatively low sulfur contents. They are often hydrocracked and isomerized to provide products with desired boiling ranges and pour point values.
Many isomerization catalysts require low levels of sulfur and nitrogen impurities, and feed streams for these catalysts are often hydrotreated to remove any sulfur and nitrogen compounds. When isomerization processes are carried out with certain non-sulfided catalysts, various side reactions, such as hydrogenolysis (non-selective hydrocracking), can occur, producing undesired C1-C4 hydrocarbons. One approach to dealing with this limitation is to suppress hydrogenolysis by incorporating a small amount of sulfur-containing compounds into the feed, or by using other hydrocracking suppressants. A disadvantage of this approach is that it adds sulfur compounds to an otherwise essentially sulfur-free composition, which may not be desired.
It would be advantageous to provide additional processes for treating Fischer-Tropsch products which minimize hydrogenolysis and which do not require the addition of sulfur compounds. The present invention provides such a process.
An integrated process for producing a hydrocarbon stream, preferably including predominantly C5-20 normal and iso-paraffin fraction, is disclosed. The process involves isolating a methane-rich stream, i.e. predominantly a C4xe2x88x92 stream, and a C5+ stream (xe2x80x9cnatural gas condensatexe2x80x9d) from a natural gas source. The methane-rich stream is treated to remove sulfur-containing impurities, if necessary, then converted to syngas, and the syngas used in a hydrocarbon synthesis process, for example, Fischer-Tropsch synthesis.
One or more fractions from the hydrocarbon synthesis are blended with the natural gas condensate such that the overall sulfur content of the blend is less than about 200 ppm. If necessary, the natural gas condensate can be treated to lower the sulfur content so that the blend has an acceptable sulfur level. In one embodiment, the fraction from the hydrocarbon synthesis is predominantly a C5-20 fraction. In another embodiment, the fraction is predominantly a C20+ fraction. In a third embodiment, the fraction is predominantly a C5+ fraction.
The blended hydrocarbons are subjected to hydroprocessing conditions. Olefins and oxygenates are hydrotreated to form paraffins. Paraffins are subjected to hydroisomerization conditions to form isoparaffins. Hydrocarbons with chain lengths above a desired value, for example, C24+, are hydrocracked. The hydroprocessing catalysts are selected for high selectivity to C5+ products in the absence of substantial amounts of feed sulfur. Thus, one or more fractions from the hydrocarbon synthesis are combined with the natural gas condensate and the hydroprocessing conditions adjusted to maximize formation of a middle distillate product. The catalyst preferred for the hydroprocessing step is a noble metal-containing catalyst, selected for high yield of desired products without the hydrogenolysis normally encountered when using base metal catalysts in a low-sulfur environment.
After the hydroprocessing step, any remaining heteroatom-containing compounds can be removed, for example, using adsorption, extractive Merox or other means well known to those of skill in the art. The processes described herein significantly reduce hydrogenolysis that would otherwise form a significant C1-C4 fraction without requiring the presence of undesired sulfur in the final product.